The present invention relates to an exhaust device for use in internal combustion engines, and a method of manufacturing such an exhaust device.
Exhaust devices, particularly manifolds for internal combustion engines have inner surfaces exposed to high-temperature, high-pressure burnt gas exhausted from engine cylinders. Accordingly, they are heated to high temperatures, permitting large amounts of thermal energy to escape. Since a recent trend is to use the high temperature of exhaust gas to achieve a higher engine performance, it is important to prevent a heat loss from the exhaust device such as a manifold.
Japanese Patent Laid-Open No. 58-51214 discloses a device such as an exhaust manifold for flowing exhaust gas from an internal combustion engine, whose inner surface is coated with a heat-insulating, refractory layer. Since the coating is formed from a slip mixture of refractory material powder, an inorganic binder and water, it must have a high water content which ensures the fluidity of the slip. This leads to shrinkage of the resulting coating layer while drying and solidifying by heat treatment, resulting in the cracking, peeling and breaking of the coating layer.
Further, when suddenly heated by a high-temperature exhaust gas, heat shock tends to provide the inner coating layer with cracks. In addition, since the refractory material powder has generally irregular particle shape, and since the exhaust device such as an exhaust manifold has a complicated inner surface structure, it is extremely hard to provide a refractory layer of a uniform thickness. A further disadvantage of this coating is that since it is made from refractory materials, it has insufficient heat insulation, despite of its good heat resistance, so that a heat easily escapes from the outer surface of the exhaust device through the inner coating layer.
Japanese Patent Laid-Open No. 58-99180 discloses a method of forming a refractory, heat-insulating coating on an inner surface of an exhaust gas-discharging device such as an exhaust manifold for internal combustion engines. This method comprises (a) casting a slip consisting of a refractory powder, an inorganic binder, a frit and water on the inner surface of a metal device which is to be exposed to a high-temperature exhaust gas to form a first heat-resistant coating, (b) attaching a refractory, heat-insulating powder onto the surface of the heat-resistant coating while the heat-resistant coating is still wet, (c) solidifying the resulting heat-insulating coating, and (d) casting a slip consisting of a refractory powder, an inorganic binder, a frit and water on the surface of the refractory, heat-insulating layer to form a second heat-resistant layer. If necessary, the above steps of forming a refractory, heat-insulating layer and a heat-resistant layer may be repeated on the second heat-resistant layer. By this method, three layers of the first heat-resistant layer, the refractory, heat-insulating layer and the second heat-resistant layer are laminated and bonded together to form an integral layer.
This method, however, forms a coating with a slip of coating materials, so the resulting coating inevitably has a relatively high water content. Thus, cracks tend to appear while drying, and large shrinkage takes place while heat treatment. Further, it is highly likely that such coating suffers from peeling and breakage. Likewise, when it is suddenly heated by a high-temperature exhaust gas, heat shock tends to generate cracks on the inner coating.
A further disadvantage of this method which uses a slip to form a heat-resistant coating is that a binder concentration in the coating is inevitably low due to the fact that the slip should have a sufficient fluidity to be coated on the inner surface of an exhaust device as uniformly as possible. A low concentration of the binder leads to an insufficient bonding between the inner surface and the coating layer as well as between the refractory powder particles, resulting in cracks and peeling of the coating.